1. Field of the Invention
The present invention relates to a light modulation apparatus for modulating a light beam two-dimensionally and outputting the same, and an optical information processing system for applying a predetermined optical process to a light beam output from the light modulation apparatus.
2. Description of the Related Art
As an optical information processing system for modulating a light beam two-dimensionally and applying an optical process to the modulated light beam, an optical information recording and reproducing system using a so-called holographic memory (hereinafter referred to as an optical memory system) is known.
The optical memory system records an input signal in a holographic memory as a recording medium as follows. A recording signal light is applied with the spatial light modulation two-dimensionally based on the input signal. For the spatial light modulation of the input signal, a transparent type spatial light modulation apparatus comprising a plurality of pixels (specifically, a liquid crystal television comprising a plurality of pixels, or the like) is used. By interference between the recording signal light applied with the spatial light modulation and a recording reference light output independently, a two-dimensionally diffracted light is produced. Then, the image formed by the diffracted light is recorded in the holographic memory as the two-dimensional information corresponding to the input signal.
As the holographic memory, for example, a volume type hologram recording element formed by laminating a plurality of lithium niobate layers is used.
On the other hand, the above-mentioned optical memory system can reproduce the input signal by reading the two-dimensional information from the holographic memory wherein the two-dimensional information is recorded. The reproducing operation is executed as follows. A reproducing reference light is directed on the two-dimensional information on the holographic memory. At the time, the reproducing reference light is directed to the holographic memory with the same incident angle as the above-mentioned recording reference light. Then, the reflected light obtained thereby is received by a light receiving apparatus having a plurality of pixels. Accordingly, a light receiving signal is obtained from each pixel of the light receiving apparatus. Then, the original input signal is reproduced using the light receiving signals.
In the above-mentioned optical memory system, in order to correctly reproduce the original input signal from the holographic memory, it is needed that the relationship in position between the spatial light modulation apparatus and the light receiving apparatus is accurately determined and fixed. Furthermore, it is needed to set the holographic memory at a precise position.
However, if a movable portion is included in the spatial light modulation apparatus or the light receiving apparatus, or if the holographic memory itself is replaced frequently, displacement or positioning error may be generated in the optical memory system. The displacement in the spatial light modulation apparatus, the displacement in the light receiving apparatus, or the displacement of the holographic memory causes the displacement between the pixels in the spatial light modulation apparatus and the pixels in the light receiving apparatus. If such a displacement is generated, the original input signal cannot be reproduced correctly.
Specifically, if the displacement is generated, reflected light corresponding to a light beam output from a certain pixel in the spatial light modulation apparatus may not be incident on the correct pixel of the light receiving apparatus. In this case, the original input signal may not be reproduced correctly.
More specifically, in the normal state without displacement, for example, it is presumed that the reflected light corresponding to the light beam output from the pixel at the i row j column in the spatial light modulation apparatus is incident on the pixel at the I.sub.1 row J.sub.1 column in the light receiving apparatus. In this case, if displacement is generated by replacement of the holographic memory or the like, the reflected light to be incident on the pixel at the I.sub.1 row J.sub.1 column may be incident on the pixel at the I.sub.2 row J.sub.2 column. As a result, a correct light receiving signal is not obtained, so that the original input signal cannot be reproduced correctly.